If you are experiencing issues with your Nokia Lumia 920, here is how to do a factory restore. Make sure to backup anything that you fear you may lose.
To hard reset hold the volume down, power button and camera button until the phone vibrates, then release the power button and keep the volume down and camera button down for another 5 seconds. Your phone should then reset to factory state. You will have to go through initial setup screen, since you bought it back to factory state.